Biological reaction processes

ABSTRACT

A process for maintaining a high rate of mass transfer of nutrients contained in a liquid feed stream to a biofilm of microorganisms supported on a bed of particulate matter. The microorganisms in the biofilm are subjected to a continuous, uniform flow of the liquid feed steam and a countercurrent flow of respiratory air so as to support biofilm growth. The bed is periodically pulsed with air at a pressure sufficient to shed microorganisms from the biofilm, and at intervals sufficient to avoid preferential channeling of the liquid feed steam through the bed and/or to disrupt and rearrange the bed of particulate matter.

FIELD OF INVENTION

This invention relates to biological reaction processes and bioreactorsfor use in such processes.

Biological reaction processes are used for the production of cell massor biomass as well as for the production of metabolites and forbiological transformations. The invention is particularly concerned withsuch processes wherein the microorganisms used in the process areadsorbed or immobilized onto a support medium.

The biological reaction process may involve the conversion of dissolvedor colloidal polluting nutrient into cell mass such as in the treatmentof sewage, industrial effluent and surface water.

For the sake of brevity and convenience, the invention will be describedin relation to a biological reaction process involving the productionand utilization of cell mass for treating a liquid feed such as sewageor industrial effluent so as to reduce substantially or eliminate thepolluting capacity of that sewage or effluent. However, it is to beunderstood that the invention is not limited to the treatment of sewageor industrial effluent as it may be applied to other feeds and to otherbiological reaction processes in which microorganisms used in theprocess are adsorbed or immobilized or otherwise supported onto asupport medium. The microorganisms (such as bacteria, fungi, algae andprotozoa) are preferably derived from the sewage and digest nutrientthat is also present in the sewage. This nutrient may containcarbonaceous material and can be in solid or soluble form, and isquantifiable in the art as Biological Oxygen Demand (BOD) and ChemicalOxygen Demand (COD). The biological reaction process can be used toremove nitrogen from sewage by conversion to nitrates.

BACKGROUND ART

Prior art biological reaction processes for the treatment of sewagesuffer from the disadvantage, to varying degrees, of low efficiency interms of the amount of resources required and the time taken to processa given volume of sewage.

Attempts to increase the efficiency of known biological reactionsprocesses for the treatment of sewage have focused on various parametersof these types of reaction processes including the means fordistribution of respiratory air to the reaction vessel; techniques forthe reorientation of the support medium to prevent channelling and tointroduce greater flow of air and liquid through the support medium; thenature of the support medium; and methods of controlling the air andliquid delivery rates to the system.

For example, Australian Patent Specification 528,760 describes a processfor purifying polluted water by percolating it downwardly through asubmerged, fixed granular bed. Oxygenated gas is fed upwardly from anintermediate level of the bed and treated water is discharged from thebottom of the bed. The flow of water to be treated and the flow of theoxygenated gas is adjusted in such a way that specific mathematicalrelationships are satisfied.

In the process described in the Australian Specification 528,760, thecritical parameters for efficient operation are said to be the rate offlow of water to be treated over the granular bed and the volume ofoxygen supplied to the microorganisms. Of these two parameters,regulation of the dissolved oxygen content of the water is the key tomaintaining the process at its optimum rate.

In order to achieve acceptable results with this process, it isnecessary to take the water through several pre- and post-treatmentstages including pre-oxidation with ozone and filtration through sand.

The process described in Australian Specification 528,760 is describedin an article by Barr K. G. in the journal Water of March 1988. In thatprocess, it is intended that the water flow/oxygenated gas flowmathematical relationships achieve high concentrations of biomass growthon the granular bed to allow high loading rates or low hydraulicretention times.

Prior art processes for the treatment of sewage or polluted water havemade a number of assumptions regarding the fundamental nature of thebiological reaction process. All these prior art processes, includingthat of Australian specification 528,760, assumed that there is arequirement for a large mass of microorganisms in order to establish alarge biofilm density to achieve increased rates of conversion ofnutrients to the desired product.

There has also been an assumption that the nutrient and environmentalrequirements (such as optimal concentration of dissolved oxygen to allparts of the system) of the microbial species play a significant part inthe efficient operation of the process.

The various elements of known biological reaction processes have allreceived investigation on the basis of the desirability for a largeamount of biofilm which is thought to result in the highest rates ofconversion and to efficient operation.

In particular, prior art processes have, in the main, tended towardsfinding ways of increasing biofilm quantity.

The improved processes arising from those investigations have met withvarying degrees of success such that the more successful processesexhibit high efficiencies in the initial stages of operation only.

However, the nature of the improvements to date have been such thatthese prior art processes have not been able to maintain the high levelof efficiency for prolonged periods. The efficiency gradually falls tounacceptable levels at which point the plant is shutdown in order thatbackwashing, cleaning, maintenance and other steps may be taken torestore the process to its initial high efficiency.

The reason for the fall in efficiency is thought to be due to theexcessively large quantity of biofilm or thick biofilm created by theprocess which clogs the system and leads to deleterious channellingeffects in the process vessel or bioreactor.

Research by the applicant has surprisingly shown that the uninterruptedgrowth of thick biofilms over long periods of time do not lead to thehighest conversion rates but to greater inefficiencies in biologicalreaction processes.

It has not been found by the present inventors that the highestconversion rate of nutrients and the highly efficient operation of theprocesses of the invention can be achieved through interrupting thegrowth of biofilm more frequently than is carried out in the prior art,and particularly in AU 528,760 and that this high efficiency can bemaintained for an equally large mass of biofilm as long as the particleschosen for the support medium are of an optimal size.

An object of the invention is to keep the rate of mass transfer ofnutrients to the microorganisms as high as possible. This is achievedlargely by minimising the detrimental effects of channelling of nutrientand air through the process vessel or bioreactor.

The mass transfer system of the invention involves the movement ofdissolved oxygen and other nutrients from the liquid feed to the biofilmand the removal of products of the biological reaction process. Thisprocess can be defined as follows:

    Nutrients+Cell Mass (microorganisms)=Cell mass +CO.sub.2 +H.sub.2 O+metabolites (products)

A high rate of mass transfer of nutrients is achieved by providingperiodic pulses of air through the bioreactor to disrupt the bed ofsupport particles and remove any localized clogging of solid matter inthe bed that may lead to channelling of nutrient away from metabolicallypotent cell mass.

The periodic pulses are preferably of an explosive nature sufficient todisrupt the bed of support particles.

The achievement of a high rate of mass transfer of nutrients is assistedby providing support particles in the bed of an optimal size. Thesmaller the effective size of the support particles, the greater is thetotal bed surface area available for attachment and growth of themicroorganisms and, accordingly, the greater the surface area ofresultant biofilm. However, as the accumulation of smaller supportparticles create smaller gaps between particles than would theaccumulation of larger support particles, the possibility of the biofilmgrowing to form a bridge across adjacent small sized particles isheightened, with the effect that effective surface area of metabolicallypotent cell mass falls. The more frequently periodic pulsing with airthrough the process vessel in the present invention removes thesebridges and restores the high effective surface area of metabolicallypotent cell mass.

In addition, the pulsing occurs at a frequency so as to control theresidence time of solids in the bed.

SUMMARY OF INVENTION

According to the invention there is provided a process for maintaining ahigh rate of mass transfer of nutrients contained in a liquid feedstream to a biofilm of microorganisms supported on a bed of particulatematter, the process comprising feeding the microorganisms in the biofilmwith a continuous, uniform flow of the liquid feed stream and acountercurrent flow of respiratory air so as to support biofilm growth,and periodically pulsing the bed with air at a pressure sufficient toshed microorganisms from the biofilm and at intervals sufficient toavoid preferential channelling of the liquid feed stream through the bedand/or to disrupt and rearrange the bed of particulate matter.

Preferably, the process is interrupted by a backwashing of the bed lessfrequently than the periodic pulsing.

The invention further provides a bioreactor for treating a liquid feedstream to remove nutrient BOD (biological oxygen demand) and COD(chemical oxygen demand) therefrom, comprising:

(i) a vessel containing a bed of particulate matter upon which grow abiofilm of microorganisms that remove nutrient BOD and COD,

(ii) means for passing the liquid feed stream downwardly through the bedand,

(iii) means for passing a continuous uniform flow of respiratory air anda flow of pulsed air upwardly through the bed.

Preferably, the means for passing air through the bed includes aplurality of spaced apart porous tubes from which the air passes intothe bed.

Preferably, the porous tubes are made of microporous polyethylene. Theparticulate material can be coal, activated carbon, anthracite, zeoliteor any inert particulate material that is capable of supportingmicroorganisms.

Preferably, the particulate material is any coal media having aneffective size of 2.3 to 2.5 mm with an uniformity coefficient of 1.5.The bed of particulate matter can be either fixed or fluidized and issubmerged during operation of the process.

The microorganisms suitable for this process are those that occurnaturally in sewage or are present in the industrial waste water to betreated. The process streams can be inoculated with microorganisms ifdesired. However, inoculation of this kind is not required in mostinstances to achieve the high operating efficiencies of the process ofthe invention.

The pressure at which process or respiratory air enters the bioreactoris not critical but is preferably between 20 and 70 kPa to provide therespiratory air requirements in an economical way.

The process of the invention may be operated in a co-current way withfluidization of the support bed. In this mode of operation, thepressures of liquid and air must be sufficient to fluidize the supportmedium in the required manner and are generally in excess of 70 kPa.

The amount of dissolved material such as nutrients in the liquid to betreated is a significant factor in the operation of the process of theinvention in as much as it effects the pulse interval and the pressureof the pulse required. Variable flow rates of the liquid to be treateddo not effect the process in any significant way.

Pulsation agitates and rearranges the bed support particles thuspreventing channelling and clumping of the support particles and thebiomass. Pulsation also removes or sheds biomass from the supportparticles.

The respiratory air creates turbulence and shear forces in the narrowvoids between particles of the bed as well as providing gaseous nutrientto the cell mass.

There is also the combined effect of the counter-current flow ofrespiratory air and the liquid to be treated which adds to theturbulence and minor agitation of the bed support particles.

The backwashing of the bioreactor vessel removes biomass shed duringoperation of the process and the sludge formed at the top of the bed.

Preferably, pulsation is performed at intervals of between 20 minutesand 2 hours. It is preferred that the pulsation is for 1 to 8 secondsduration. Preferably, the pressure of the pulses is in the range of 60kPa to 120 kPa, but more preferably 70 kPa. The effect of the pulsationis explosive in nature, causing rapid disruption of the bed.

It is preferred that the backwashing of the bioreactor vessel be carriedout at intervals of from 3 to 24 hours. It is further preferred thatbackwashing be carried out for a duration of from 3 to 7 minutes.

Preferably, the backwashing regime involves stopping the operation ofthe biological reaction process and pumping liquid upwardly through thebioreactor vessel at a rate sufficient to cause the support particles tolift uniformly and gently.

During the pumping of liquid, pulses of air are sent upwardly throughthe vessel to shed biomass from the support particles. The lightersewage solids and shed biomass are left at the top of the vessel and aredrained off with the liquid containing those solids and shed biomass.

In a particular form of the invention, the bioreactor vessel isinterconnected with a continuous microfiltration unit of the typedescribed in Australian Patents 563,321 and 576,424 whereby allremaining solids and shed biomass are removed from the bioreactor vesseleffluent by the filtration unit and the effluent is disinfected. Thebackwash from the continuous microfiltration unit may be recycled intothe bioreactor feed.

Preferably, the liquid that is pumped upwardly during the backwash cycleis effluent from the bioreactor.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings in which:

FIG. 1 is a perspective view of a bioreactor according to a preferredembodiment of the invention,

FIG. 2 is a front elevational view of the bioreactor of FIG. 1,

FIG. 3 is a side elevational view of the bioreactor of FIG. 1,

FIG. 4 is a sectional view taken in the direction of arrows AA of a partof the bioreactor shown in FIG. 2,

FIG. 5 is a sectional view taken in the direction of arrows BB of FIG.2,

FIG. 6 is a plan view of the bottom section of the bioreactor of FIG. 1,

FIG. 7 is a front elevational view of the bottom section of thebioreactor of FIG. 1,

FIG. 8 is a sectional view taken in the direction of arrows CC of FIG.7,

FIG. 9 is a sectional view taken in the direction of arrows DD of FIG.8,

FIG. 10 is a plan view of the top section of the bioreactor of FIG. 1,

FIG. 11 is a side elevational view of an air supply header for thebioreactor of FIG. 1,

FIG. 12 is a sectional view taken in the direction of arrows EE of FIG.11,

FIG. 13 is a sectional view taken in the direction of arrows FF of FIG.12,

FIG. 14 is a side elevational view of a liquid flow header for thebioreactor of FIG. 1,

FIG. 15 is a sectional view taken in the direction of arrows GG of FIG.14,

FIG. 16 is a sectional view taken in the direction of arrows HH of FIG.15,

FIG. 17 is a schematic diagram of a sewage treatment plant utilizing thebioreactor of FIG. 1,

FIG. 18 is a graph of flow rate against time for a bioreactor operatedin accordance with the pulsing techniques of the present inventioncompared with a bioreactor operated without the pulsing techniques,

FIG. 19 is a graph of suspended solids concentration against time for abioreactor operated in accordance with the pulsing techniques of thepresent invention invention compared with a bioreactor operated withoutthe pulsing techniques,

FIG. 20 is a graph of the suspended solids concentration time profileafter the bioreactor has been pulsed in accordance with the principlesof the invention,

FIG. 21 is a graph of the effluent COD against COD loading rate for abioreactor operated in accordance with the pulsing techniques of theinvention, and

FIG. 22 is a graph of the effluent COD against time for a bioreactoroperated in accordance with the pulsing techniques of the inventioncompared with a bioreactor operated without the pulsing techniques.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The bioreactor 10 shown in FIGS. 1, 2 and 3 includes a vessel 11 ofrectangular prism shape having a front wall 12, two identical side walls13 and 14 and a rear wall (not shown) identical to the front wall. Eachwall has a series of stiffening flanges 16. The cross-sectional area ofthe vessel 11 is, in this instance, approximately 20 square meters.

The vessel 11 has sight glasses 17 for visual inspection of the interiorof the vessel 11 and lifting beams 18 on its front, side and rear walls.

During normal operation of the bioreactor 10, air is supplied to theinterior of the vessel 11 through air supply headers 19, that extendalong the front and rear walls, and liquid is withdrawn from theinterior of the vessel 11 through liquid flow headers 20 also extendingalong the front and rear walls. The headers 19 and 20 have hinged covers26 and 27 for access to the main header air supply tube 43 and mainheader liquid flow tube 44 therewithin. Handles 33 and 34 facilitate theopening of the covers 26 and 27.

Located within the vessel 11 is a packed bed of coal particles 21 (shownin part through a removed portion of the front wall 12 in FIG. 2) ofeffective size between 2.3 and 2.5 mm with a uniformity co-efficient of1.5. The bed particles 21 rest on the floor 22 of the vessel 11.

A central stiffening flange 16a divides the top section 23 from thebottom section 24 of the vessel 11. A plan view of the top section 23 inisolation is shown in FIG. 10. Plan and front side views of the bottomsection 24 in isolation and with the headers 19 and 20 removed andinternal tubes removed are shown in FIGS. 6 and 7.

Drain ports 21 located on both side walls 13 and 14 allow the drainingof liquid and the removal of bed particles from the vessel 11.

The vessel 11 rest on a series of stands 25 connected to the vesselfloor 22. The floor 22 has drain channels 22a, shown in FIG. 5, thatlead to the drain ports 21.

The vessel 11 is divided into six identical treatment compartments orcells 35, 36, 37, 38, 39, and 40 that are separated by dividing wallsshown in FIGS. 5 and 10.

The air supply header 19 and liquid flow header 20 are shown in FIG. 1providing access to a plurality of vessel air supply tubes 28 thatcommunicate with the main header air supply cavity 43 and the vesselliquid collector tubes 29 that communicate with the main header liquidflow cavity 44 defined within their respective headers. FIG. 11 showsthe air supply header 19 in isolation.

Backwash exit ports 31 and 31a are located at both side walls 13 and 14and communicate with the interior of the vessel 11 through a trough andsyphon arrangement located in the top section 23 of the vessel. Thetrough and syphon arrangement is described in more detail with referenceto FIGS. 5 and 10.

Liquid feed troughs 32 and 32a extend along the length of the opposedinside upper ends of the vessel 11. The liquid feed troughs 32 and 32aare described in more detail with reference to FIGS. 5 and 10.

FIG. 4 shows a longitudinal cross-sectional view of an individual vesselair supply tube 28 and an individual vessel liquid collector tube 29.Both tubes are close ended against the central dividing wall 41. Vesselair supply tube 28 has an opening 42 to the main header air supplycavity 43 within the air supply header 19. Vessel liquid collector tube29 has an opening 45 to the main header liquid flow cavity 44.

Each of the vessel air supply tubes 28 and vessel liquid collector tubes29 is enclosed in a stainless steel mesh coal particle shield 30 and 30arespectively (shown in cross sectional detail in FIG. 4).

For the vessel air supply tubes 28, the particle shield 30 is enclosedaround an inner tube 46 of microporous polyethylene. Air is able to passthrough the pores of the polyethylene tube 46 and the mesh shield 30.For the vessel liquid collector tubes 29, the particle shield 30a isenclosed around an inner tube 47 made of ABS polymer. The inner tube 47has a series of arcuate slits 48 spaced about 40 mm apart along itslength that are cut part-circumferentially around its lower portion.Each slit 48 has an opening width of about 0.5 mm.

FIG. 5 shows a sectional view through the vessel 11 to reveal theinterior of the compartments 40 and 37. These compartments are separatedfrom each other by the wall 41 and are separated from the compartments39 and 36 respectively, which are behind them, by dividing walls 49 and50. Walls 49 and 50 are perforated by holes 51 that enable thecompartments 40 and 37 to be in liquid communication with compartments39 and 36 respectively, which in turn are in liquid communication withcompartments 38 and 35 respectively through similar holes in walls 52and 53. The dividing wall 41 between the set of compartments 35, 36 and37 and the set of compartments 38, 39 and 41 does not have anyperforations and therefore these two sets of compartments are not inliquid communication with each other and can operate independently ofeach other.

The central dividing wall 41 has a bifurcation point 54 where the wall41 divides into subwalls 55 and 56. The subwalls 55 and 56 of wall 41define a trough 57 therebetween. The trough 57 has exit ports 31 and 31athrough the side walls 13 and 14 respectively.

Located in the top section of each vessel compartment are a pair ofsyphons. Syphons 58 and 59 of compartments 40 and 47 respectively areshown in FIG. 5. Each siphon 58 and 59 is supported by a clamp arm 60and 61 respectively mounted to the front and rear vessel wallsrespectively. Each syphon 58 and 59 has a trough opening 62 and 63respectively through the subwalls 55 and 56 respectively against whichsubwalls the syphons are also supported. At the other end of each syphon58 and 59 is a bell mouth opening 64 and 65 respectively.

The bed particles are packed to a height about 100 to 200 mm below thebell mouth openings 64 and 65

FIG. 10 shows a top view of a syphon 66 located in compartment 35supported by clamp arm 67.

Although not shown in FIG. 10, each compartment has two syphons.

The liquid feed troughs 32 and 32a shown in FIGS. 5 and 10 are closed atboth ends. Each trough 32 and 32a has an upstanding knife edge wall 68and 69 respectively over which liquid feed filling each trough isadapted to spill into the vessel 11.

The walls 68 and 69 have spaced apart V-shaped notches 70 that regulatethe volume of liquid feed that will spill into the vessel 11.

Liquid feed is fed into the troughs 32 and 32a through hose pipes (notshown) having their outlets resting within the troughs.

The vessel bottom section 24 shown in FIGS. 6 and 7 has had the headers19 and 20 and vessel air supply tubes 28 and vessel liquid collectortubes 29 removed therefrom. The lifting beams 18 have also been removed.The openings in the front wall 12 adapted to communicate between themain header air supply cavity 43 and the plurality of vessel air supplytubes 28 are shown as openings 42a. The openings in the front wall 12adapted to communicate between the main header liquid flow cavity 44 andthe plurality of vessel liquid collector tubes 29 are shown as openings45a.

FIG. 8 shows a sectional view through the bottom section 24 of FIG. 7 toreveal the dividing walls 41, 49 and 50 and holes 51 therein. Also shownare the wall flanges 70 for both of the headers 19 and 20, and sightglasses 17.

FIG. 9 shows a sectional view through the bottom section 24 of FIG. 8 toreveal the dividing walls 41, 49 and 52. Receiving pipes 71 extendingfrom the wall 41 are adapted to support the ends of the vessel airsupply tubes 28. Similar receiving pipes 72 are adapted to support theends of the vessel liquid collector tubes 29.

The air supply header 19 shown in isolation in FIG. 11 has three hingedcovers 26, 26a and 26b and handles 33 and is shown sectionally in FIGS.12 and 13. The main header air supply cavity 43 defined within theheader 19 has end openings 73 and 74 and stiffeners 75.

The liquid flow header 20 shown in isolation in FIG. 14 also has threehinged covers 27, 27a and 27b and handles 34 and is shown sectionally inFIGS. 15 and 16. The main header liquid flow cavity 44 defined withinthe header 20 has end openings 76 and 77 and stiffeners 78.

The packed bed of coal particles within the bioreactor vessel 11supports the growth of microorganisms preferably derived from the sewageor other feed to be treated. The microorganisms digest nutrient in thefeed and, as a result, reduce BOD and COD. The growth of themicroorganisms on the particulate material forms a film ofmicroorganisms or biofilm on the particles.

During normal operation, the microorganisms in the bed are supplied withrespiratory or process air containing oxygen that is introduced underslight pressure through the main header air supply cavity 43 and theninto the series of vessel air supply tubes 28 from where it escapes intothe vessel 11 through the pores of the microporous polyethylene materialmaking up the tubes 28. The process air enters the main header airsupply cavity 43 through one end thereof only (shown by arrow A inFIG. 1) and the opposite end is closed by an appropriate valve mechanism(not shown) to allow build up of the desired pressure.

At predetermined intervals, pulse air is introduced under higherpressure through the opposite end (shown by arrow B in FIG. 1) of themain header air supply cavity 43 to that used for introducing processair. The now opposite end is closed by an appropriate valve mechanism(not shown) to enable the required pulse pressure to be generated.

During normal operation, treated effluent leaves the vessel 11 bypassing through the slots 48 into the vessel liquid collector tubes 29,passing through the tubes 29 to the main header liquid flow cavity 44and then leaving the cavity 44 in the direction of arrow C shown inFIG. 1. The opposite end of the cavity 44 is closed by an appropriatevalve mechanism (not shown).

At predetermined intervals, treated effluent is backwashed into thevessel 11 through the slots 48. The backwash liquid enters the mainheader liquid flow cavity 44 in the direction of arrow D shown in FIG.1, before passing to the vessel liquid collector tubes 29 from where itis expelled into the vessel 11. During the backwash operation, theopposite end of the cavity 44 is closed by an appropriate valvemechanism (not shown).

The operation of the bioreactor 10 in the sewage treatment plantdepicted schematically in FIG. 17 will now be described.

Raw degritted sewage passes into a primary setting tank 135 and allowedto settle. The supernatent therefrom is pumped by feed pumps 136 throughhose pipes into the feed troughs 32 and 32a of the bioreactor 10 fromwhere it spills onto the liquid already submerging the bed.

The bioreactor vessel contains a bed of coal particles 2.8 m in height,giving an effective bed volume of 56 m³. The bed is submerged in 26 m³volume of sewage which during normal operation is at a height of between0.1 to 1.2 m above the bed. The ratio of void volume to coal volume ofthe bed is about 0.4. The sewage temperature is 20° C.

During normal treatment operation, sewage flow rate through thebioreactor is 100 m³ /hr. The countercurrent respiratory air flow rateis 240 m³ /hr.

The metabolic activity of the microorganisms leads to cell growth andproliferation through rapid asexual division and this, in turn, leads toan increase in the thickness of the biofilm on the bed particles.

In order to limit the growth of the biofilm to a thickness where themicroorganisms comprising the biofilm do not clog the bed, the bed isperiodically subjected to an upward air pulse.

This normal treatment operation was continued for 60 minutes, whereaftersewage feed and respiratory air flow was stopped in compartments 38, 39and 40 but continued in compartments 35, 36 and 37. A 2 second delayfollows to allow valves to close.

In compartments 38, 39 and 40, an air pulse lasting 3 seconds at 70 kPapressure was then passed through the bed. This was followed by a secondidentical air pulse 8 seconds later. During each air pulse, 5000 litersof air was injected into the bed. After the second pulse, sewage feedand respiratory air flow in compartments 38, 39 and 40 were resumed atthe previous rates for another 60 minutes until the pulse operation wasrepeated. Air pulsing was also carried out in compartments 35, 36 and 37with the same parameters as above but at different times to the pulsingof compartments 38, 39 and 40 so that all six compartments are notpulsed simultaneously.

The cycles of treatment operation and pulsing were continued for aperiod of 8 hours until commencement of the backwashing operationwhereupon the treatment operation in compartments 35, 36 and 37 wasstopped. An air pulse identical to previous pulses was then pulsedthrough the bed, 12,000 liters of filtrate from the bioreactor waspumped backed into the compartments 35, 36 and 37 at a flow rate of 380m³ /hr. For the first 80 seconds successive mini-pulses of air werepumped into the se compartments as before but of only 1 second durationand spaced 10 seconds apart.

After the pumping of the filtrate and succession of mini-pulses hasceased the respiratory air is restored for 60 seconds. This restores airto the bed and ensures that the particles in the bed repack to form thebed. The respiratory air is then removed and a further 40 secondsettling period allows coal particles that have been suspended in thebackwash effluent to settle. After the settling period, the sewage feedis reapplied for a period of 30 seconds until the syphon operationstarts.

The backwash liquor is then removed through the syphons from the top ofthe bioreactor. This syphoning process was completed within 135 secondswhereafter normal treatment operation and pulse cycles were resumed. Theentire backwashing operation took 6 minutes 34 seconds in this instance.

During normal treatment, the treated sewage that leaves the bed throughthe slots 48 passes via the header 20 to a balancing tank 137 beforebeing filtered by a continuous microfiltration unit (CMF) 138 operatedaccording to Australian Patents 563,321 and 576,424. The teachings ofAustralian Patents 563,321 and 576,424 are incorporated herein byreference.

Clear, disinfected water is produced after the further treatment by theCMF. Liquid from the balancing tank 137 may be diverted to the storagetank 139 to be used in the bioreactor backwash. Solids collected on themembrane filter not shown) of the CMF unit 138 would periodically beremoved, by the CMF backwash, to the settling tank 135 and supernatentliquid would then be pumped into the bioreactor 10. The backwash of theCMF 138 is effected primarily by using air blowback.

In backwashing shed biomass from the bioreactor 10 the backwash liquidis pumped in at the bottom of the bioreactor 10 and liquid carrying shedbiomass is collected through the siphons at the top of the bioreactor10. The solids thus collected are settled for a given period in settlingtank 135. The settled solids are then sent to a sludge thickener 141 anda dewatering plant 142 for further concentration and processing intostable sludge 143, whereas the clearer liquid taken from the settlingtank 135 is ultimately recycled back to the bioreactor 10. Air forrespiratory, pulse and backwash operations of the bioreactor 10 and forblowback of the CMF 138 is supplied by an air delivery system 144.

In the present invention the frequency at which pulsing is effected isgreater than in prior art systems which, in the main, have focused onlong term stability of the bed for relatively uninterruptedmicrobiological metabolic activity. As foreshadowed earlier, thebioreactor beds of the prior art rapidly become clogged by biomass asuninterrupted biofilm growth bridges the gaps between bed particles andleads to longer sewage residence times.

The effect of the periodic pulsing with air in the present invention isto reorientate or mix the bed particles, so removing any channellingeffects on respiratory air or the sewage stream. As channelling may leadto preferential passage of nutrient to some areas of the bed only,whilst leaving other areas of the bed starved of nutrient, with theresult that the effective bioreactor bed volume is reduced, eliminationof channelling effects is an important element in the efficientoperation of any bioreactor.

The importance of the explosive pulsing regime in controlling sewageflow rate through the bioreactor of the invention by maintaining anunclogged bed is shown in the following example. The example was carriedout using two 6 inch diameter cylindrical trial bioreactors in parallelto determine the effect of pulse and no pulse on bioreactor performance.The intention of the trial was that the only difference between theoperation of the two bioreactors would be the pulse and no pulse.

A media depth of 3 meters was used. The coal media used as the biofilmsupport media had an effective size of 2.3-2.5 mm with a uniformitycoefficient of 1.5.

These trials were carried out for 25 days. Both bioreactors werebackwashed after 8 hours with the pulsed reactor being regularly pulsedevery 60 minutes with two short pulses of air through the air supply.These pulses are typically of 3 seconds duration. Normal aeration isfrom 10-25 m³ hr⁻¹ per m² of bed with the pulse air being supplied at600 m³ hr⁻¹ per m² of bed. Temperature of the feed varied from 16°-22°C. The no pulse bioreactor was visibly more difficult to control. After8 days the backwash needed to be changed and more air used in thebackwash in order to keep it running.

This can be seen by comparing the flows of the two bioreactors in FIG.18.

The biofilm growth on the non pulsing bioreactor was not controlled andso led to a need to have a more vigorous backwash in order to controlthe media. This can also be seen in the suspended solids in the backwashof the two bioreactors as shown in FIG. 19.

A typical pulse profile of the suspended solids in the effluent is shownin FIG. 20.

The volume of the bioreactor backwash was 351 and so the difference inmass between pulsing and no pulsing is 42 g. In the immediate aftermathof a pulse as shown above 2 g of solid are removed which corresponds to16 g in a backwash interval. The remainder of the difference will beremoved in the rest of the pulse and also due to natural variation.

The storage capacity of the current system being used is calculated at1.5 kg/m³.

The use of pulsing thus allows control of the residence time of thesolids within the bed. Backwash can be reduced but not necessarilyeliminated as incoming suspended solids may tend to congregate near thesurface of the bed.

A larger surface area per unit volume allows higher loading rates. Theloading rates achieved in the bioreactor trial are shown in FIG. 21.

These results compare with unpulsed technology as currently used in theprior art of Australian Patent 528,760 which applies a maximum load of 7kg CPD/m³.

Microporous aeration tubes are a good method of distributing air into abioreactor. They have good distribution properties and produce a finestream of bubbles. They have however a tendency to become blocked withuse. If these are pulsed regularly with air then the tendency to blockis reduced. This is shown in FIG. 22.

Various modifications may be made in details of design and constructionof the bioreactor and in details of the biological reaction process ofthe present invention without departing from the scope or ambit of theinvention.

We claim:
 1. A process for maintaining a high rate of mass transfer ofnutrients contained in a liquid feed stream to a biofilm ofmicroorganisms supported on a bed of particulate matter, said processcomprisingA. feeding said microorganisms in said biofilm with acontinuous, substantially uniform flow of said liquid feed stream and acountercurrent flow of respiratory air so as to support biofilm growth,and B periodically pulsing said bed, while sustaining said continuous,substantially uniform flow of said liquid, with air at a pressure andfor a time sufficient to shed at least an increment of saidmicroorganisms from the biofilm and/or to disrupt and rearrange the bedof particulate matter and at intervals sufficient to preventpreferential channelling of the liquid feed stream through the bed. 2.The process of claim 1 wherein the process is interrupted by a liquidbackwashing of the bed less frequently than the periodic pulsing.
 3. Theprocess of claim 2 wherein the bed is backwashed at intervals of 3 to 24hours.
 4. The process of claim 2 wherein the bed is backwashed for 3 to7 minutes.
 5. The process of claim 2 wherein the bed is backwashed withtreated feed stream effluent.
 6. The process of claim 1 wherein therespiratory air enters the bed at a pressure of between 20 kPa and 70kPa.
 7. The process of claim 1 wherein the bed is pulsed at intervals ofbetween 20 minutes and 2 hours.
 8. The process of claim 1 wherein thebed is pulsed for 1 to 8 seconds.
 9. The process of claim 1 wherein thepulsed air enters the bed at a pressure of between 60 kPa and 120 kPa.10. The process of claim 9 wherein the pressure is 70 kPa.
 11. Abioreactor for treating a liquid feed stream to remove nutrient BOD andCOD therefrom, comprising:A. a vessel containing a bed of particulatematter upon which grow a biofilm of microorganisms that remove nutrientBOD and COD; B. means for passing the liquid feed stream downwardsthrough the bed; and C. means for passing a continuous uniform flow ofrespiratory air and for producing a pulsed flow of air upwardly throughthe bed at a pressure and for a time sufficient to shed microorganismsfrom the biofilm and/or to disrupt and rearrange the bed of particulatematter, and sufficient to avoid or eliminate preferential channeling ofthe liquid feed steam through the bed.
 12. The bioreactor of claim 11wherein the means for passing air through the bed includes a pluralityof spaced apart porous tubes from which the air passes into the bed. 13.The bioreactor of claim 12 wherein the porous tubes are made ofmicroporous polyethylene.
 14. The bioreactor of claim 11 wherein theparticulate matter is any coal media having an effective size of 2.3 to2.5 mm with a uniformity coefficient of 1.5.
 15. The bioreactor of claim11 wherein the bed is a fixed bed and is submerged during the treatmentof the liquid feed stream.
 16. A liquid feed stream treatment plantcomprising the bioreactor of claim 11 and a microfiltration unit toremove solids from the effluent of the bioreactor.